I think what she is doing while on top is hitting her G spot. This is where she is getting a lot of pleasure probably.
You can angle your self to hit it some times.
I would suggest on next time doing normal missionary position and putting her legs on your shoulders while having sex. That usually helps you hit the G spot. Just ask her to try it, and make sure shes OK while having sex and that she is not uncomfortable.
How You Can Finally Spot The G-Spot
